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Mission Critical Fuel  - What is Diesel Bug?

Diesel fuel held in long term storage is susceptible to microbial contamination. This contamination is caused by a consortium of bacteria, yeast and fungi that can grow in your diesel fuel storage tanks. 

Microbial contamination, often commonly called “the Diesel Bug” can damage tanks, fuel lines, pumps and injectors as well as result in poor fuel economy and excessive emissions.  However, the greatest potential for damage to businesses that rely on fuel storage for Standby and Emergency Generators is blocked fuel filters and unexpected engine shutdowns, particularly following a mains power outage.

What causes the onset of microbial contamination? 

There are three elements needed to complete the eternal triangle and enable microbial activity to occur and contamination to build. 

Understanding Microbial Contamination 
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Most, if not all microorganisms arrive in a tank as a result of travelling down the fuel supply chain from tank to tank. The microorganisms (bugs) in the fuel use the fuel itself along with the fuels additives as a food source. The water, once present in the tank provides life. In any tank once the eternal triangle is complete microbial growth (biomass) is inevitable. 

Gensets are typically only test run for 30 minutes a month, which means that the amount of contamination drawn through into the primary filter sight bowl can be so small that the contamination often remains unseen. The problems arise when the engine is called upon to run in earnest for many hours. It is during this extended run that you will be drawing through much more contamination along with the fuel. It’s at this critical stage that filters can become blocked and may result in a shutdown. 

H20 and the problems it causes: 

When water is allowed to build up in fuel storage tanks it provides an ideal environment for microbial growth to occur. Sample on the right contains rusty water with a small amount of fuel sitting on top. 

While water ingress and heavy microbial infections are not uncommon they only usually occur as a result of inaction or lack of preventative measures. If any contamination, however slight, can be seen in primary filter sight glasses or water collection bowls then it is usually too late as the infection will have already taken hold in the bottom of the tank. It only takes very small amounts of water (droplets in fact) to kick-start microbial activity. It is imperative that any water is kept at bay. In Standby fuel storage applications water can ingress into a tank in a number of ways – these being condensation, leaking seals or from a contaminated fuel supply source. 

Condensation is caused by tanks breathing in moist air and allowing it to condense on the walls. It can also occur when there is a temperature differential between the inside and outside of a tank e.g. at night or after an engine has been running for long periods of time and pushing back hot fuel via the fuel return line into the tank. The more vacant wall space available, the more water can and will condense and build up in the tank. 

Rain water, flooding and/or wash down water can enter tanks via leaking fuel fill points, broken pipework, old seals or even poorly located tank breathers. Underground tanks are particularly vulnerable for this type of water ingress. 

Water can also come with the fuel supplied. While it is hard to imagine your fuel supplier is also susceptible to exactly the same problems. While their procedures and maintenance are usually of a very high standard from time to time issues do occur and contamination can be transferred unknowingly along with the fuel. 

Water that enters or builds up in the fuel can drop out of phase, run down the walls and eventually build up in the bottom of the tank as ‘Free Water’. While some Free Water may get picked up and separated out in the primary fuel filter (if there is one) it is not normally the case as fuel pickups are generally well above the tank floor. This unfortunately means the water can sit in the bottom of a tank for a very long time undetected, and thus provide the perfect environment for microbes to grow and build biomass/sludge. 

Aside from aiding microbes water also causes corrosive (rust) damage to tank surfaces and fuel systems. Heavy rust scaling can often be found in flat bottom tanks in particular. 


There are some 125 microbes, bacteria and fungi (yeasts and moulds) that have the ability to grow in middle distillate fuels such as diesel and aviation fuel. Around 30 out of the original list of 125 have the potential to cause problems in diesel with one in particular being very troublesome. Depending on the conditions present in the tank, various microbes will grow at different rates. This makes the fuel particularly vulnerable to contamination during prolonged storage, which is why the standby and emergency power generation industry is hit so hard with the problem. 

Bacteria, yeast and fungi all grow in the ‘Free Water’ under the fuel, most of which can be seen at the interface between the water and the fuel phases. Microbes are also able to grow throughout the fuel reticulation system and even on the downstream side of filters. 

While microbes feed off the Alkanes (hydrocarbons) in the fuel it is the water that gives them life. Many of the additives in fuel also provide nutrients, which also aids growth with most of the visible contamination occurring, as mentioned at the interface. This is something that can clearly be seen when large amounts of ‘Free Water’ are present in the fuel. Microbial growth can also occur on hard to see tiny droplets of water that are suspended in the fuel. The outside of each droplet acts as the interface between the water and fuel. 

With modern HPCRD engines (High Pressure Common Rail Diesels) it is this suspended water carried through by the microbes that causes most of the problems. It can cause damage to hugely expensive lift pumps and injectors, even if the correct filtration is in place. These modern engines have very tight tolerances so struggle to handle any level of contamination in the fuel. 

Fuel Phase – When cloudy as shown emulsified/suspended water is also present in the fuel. 

Interface – The bulk of the microbes (bugs) live at the interface between the fuel and the water. 

Free Water Phase – Always at the bottom as water is heavier then fuel. There can be up to a 1000 times more bugs in the water then in the fuel itself. 

What is “Diesel Bug”?

The most troublesome microbe that has the ability to adhere to the tank surfaces and cause most damage is the fungal organism called Hormoconis resinae (H.res) – previously known as Cladosporium resinae. It’s this particular microorganism that is actually known as the “Diesel Bug” or in the aviation industry as the “Jet Fuel Fungus”. This H.res microbe is also responsible for the damage that can occur to alloy and steel tank surfaces. It has the ability to electro-chemically drill down into the metal substrate and cause pitting corrosion. The bug can also survive freezing and thawing and very dry conditions. 

Best Practice Procedures: 

There are a number of Best Practice Procedures that should be utilised in a good Fuel Management Programme to reduce the risk of water ingress and microbial growth. 

These include: 

  • Where possible all tanks should be kept above 80% full at all times. Top up’s should be done regularly. This prevents condensation building up on the tank walls. 

  • Utilising drain valves where possible to regularly drain off small amounts of fuel and any water from the bottom of tank or from dedicated low points if the tank has them. 

  • Inspect seals, pipework, man-ways (inspection hatches) and breathers regularly for any signs of damage or deterioration. Replace or repair if required. 

  • Only take fuel from reputable sources or suppliers. When large volumes of fuel are being supplied ask for a sample before accepting the fuel. 

  • Fit moisture traps to the breathers on the tank. These filter the movement of air and prevent moisture from being drawn down into the tank. 

  • Sample and test the fuel from the bottom of the tank on a regular schedule basis. 

  • Look at installing an automated fuel polishing system. 

  • Instigate a Fuel Management Programme as a part of your PPM. 

Sample on the right contains rusty water with a small amount of fuel sitting on top. 

This black and dark organic material is a combination of bacteria, yeast and the H.res fungi. 

An example of corrosion in a stainless tank. 

Microbial material can be drawn up by the fuel pick-up lines. It is then transferred and deposited in the primary filter. By the time it can be seen, many tens of thousands, if not hundreds of thousands of spores have joined together to make what appears to be black spots, stringy matter or watery sludge in the filter sight bowls. If this material makes its way past the filters it can also result in extensive damage and costly repairs to fuel pumps and injectors. While this shouldn’t happen with quality filtration in place it’s amazing what these little mighty microbes can do.

At the start of an infection it is one of the most progressive as it has the ability to produce copious amounts of biomass. As the infection grows other fungi like Yarrowia lypolitica (a common yeast) take over by out-competing for the same food source. This usually results in the H.res dying out, leaving the dead material either floating around or stuck to the tank surfaces, often with considerable damage obscured beneath. 

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Some microbes can hold water in suspension while others have the ability to produce water themselves. 

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Fuel Management Programmes: 

Fuel Management Programmes including monitoring for fuel quality should be put in place for all tanks that support Mission Critical Equipment. Sampling and testing from the bottom of the tank on a regular, scheduled basis offers the best opportunity for detecting Free Water and microbial activity. On larger bulk tanks samples from the top and middle of the tank should also be considered. While chemical testing for suspended water and other important components tells you if the fuel is within spec (i.e. meets Government Regulations) only a full microbial laboratory test will determine if the fuel is “Fit for Purpose”. 

If you have any questions, require specific advice on your tank systems or would like Pacific Fuel to put in place a Fuel Management Programme then please don’t hesitate to contact us. 

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